1. Field of the Invention
The present invention relates to an up-down tuner for use mainly in home terminals, CATV devices for multimedia use, TVs, and VTRs.
2. Description of the Prior Art
FIG. 9 is a block diagram of a prior art up-down tuner. FIG. 10 is a block diagram of a generic tuner. FIGS. 11 and 12 are plan views showing concrete structures of the up-down tuner of FIG. 9 and the tuner of FIG. 10, respectively.
In FIGS. 9 and 10, BPF denotes a band-pass filter, AMP denotes an amplifier, MIX denotes a mixer, OSC denotes a local oscillator, and PLL denotes a phase locked loop.
In general, an up-down tuner has two local oscillation circuits unlike a generic tuner, as is evident by a comparison of FIG. 10 with FIG. 11. Therefore, the up-down tuner tends to incur a slight deviation of an oscillation frequency and a spurious interference due to an input signal and two local oscillation signals. The spurious interference is defined as follows. In an electronic radio wave radiated to the space from a transmitter or another electronic device via an antenna, there are included radiation waves of unnecessary frequencies other than frequencies in the necessary bandwidth (occupied bandwidth). The unnecessary radiation waves are called "spurious". The spurious includes higher harmonics and fractional harmonic waves of a wave of a transmission frequency, a fundamental wave for frequency modulation, and higher order sideband wave frequencies. When an output stage of a transmitter has an insufficient filter characteristic for tuning, a considerable amount of spurious is radiated from the antenna, often causing interference with other communication lines. This is referred to as spurious interference. That is, in up-down tuners, the spurious interference is principally a mutual interference of two local oscillation signals in the up-down tuner.
Heretofore, a countermeasure has been taken by providing a metal chassis with an increased number of shield portions in an interior area thereof, partitioning circuits, and providing an increased number of grounding portions (soldered portions).
Practically, as shown in FIG. 11, the prior art up-down tuner is provided with a printed wiring board 1 for high-frequency components and the like, and a metal chassis 3 penetrating a slit 2 formed in the printed wiring board 1, where the printed wiring board 1 is connected by solder with the metal chassis 3 at copper foil portions (not shown) of the printed wiring board 1. In order to improve the reliability of the soldered portions of the metal chassis 3 and the printed wiring board 1, an increased amount of solder is deposited between the copper foil portions of the printed wiring board 1 and the metal chassis 3 by sufficiently amassing solder (reinforcement soldering) by means of a soldering iron or by mounting dummy chips or the like.
In FIG. 11, a reference numeral 3a denotes a shield portion of the metal chassis 3, and a reference numeral 4 denotes solder.
In FIG. 11, there are shown an RF (Radio Frequency) section 11, a first local oscillation section 12, a first local amplification section 13, a first local PLL (Phase Locked Loop) section 14, a first mixer section 15, a first IF (Intermediate Frequency) section 16, a second mixer section 17, a second local oscillation section 18, a second local PLL section 19, and a second IF section 20. As a countermeasure against the spurious interference, the above-mentioned sections are partitioned by the shield sections 3a of the metal chassis 3 and an increased number of soldered portions are provided between the printed wiring board 1 and the metal chassis 3. Circuit structure of above-mentioned sections is as shown in FIG. 9.
For comparison, the structure of the prior art generic tuner is shown in FIG. 12. In FIG. 12, components similar to those shown in FIG. 11 are denoted by same reference numerals.
FIGS. 13A, 13B, 13C and 13D are explanatory views of solder connections or joints of the prior art up-down tuner. FIG. 13A shows a solder joint achieved by solder dipping, where the solder dipping forms a thin solder joint 4 which tends to generate a crack. FIG. 13B shows a solder joint achieved by bending a part of the metal chassis 3, according to which a certain amount of solder can be secured. However, the solder joint 4 is still thin and generation of a crack is possible. FIG. 13C shows a solder joint achieved by amassing solder (reinforcement soldering) using a soldering iron, according to which generation of a crack can be prevented. In addition, FIG. 13D is a solder joint achieved through the use of a dummy chip 5, according to which a sufficient amount of solder can be secured to allow the prevention of the generation of a crack. Therefore, normally the printed wiring board 1 and the metal chassis 3 are soldered to each other by securing a sufficient amount of solder through amassing solder or mounting dummy chips or the like.
However, the measure of amassing solder or mounting dummy chips 5 which the above-mentioned prior art up-down tuner uses for the improvement of reliability, have caused a significant cost increase due to a troublesome soldering operation and the use of the dummy chips.
Furthermore, the metal chassis 3 has a cutout portion for supporting the printed wiring board 1. Therefore, the cutout portion is ineffective in shielding and consequently exerts influence on the spurious interference characteristic.